Since most CPUs and Heatsinks are not perfectly flat (even when lapped), the thermal paste (if only added at the center) will only spread to cover a portion of the IHS.... although the majority of the heat from the core DOES concentrate at the center of the IHS, some of the heat also finds itself at the edges of the IHS (where there is a gap and no contact due to a lack of thermal paste)....

Although the general argument is that less thermal paste = more contact between the ACTUAL surfaces... I believe that pre-spreading would allow for a contact between the unavoidable gaps at the edges of the CPU's IHS and Heatsink.... and therefore would allow for more heat transfer to the HSF

Well, yes and no. The problem with the spread method is that you can introduce little air bubbles if you're not careful. I find your best bet is a line of thermal paste that is just enough to cover the IHS when squished down.